Chapter 19 Organic Chemistry and Hydrocarbons part 1

Chapter 19 Organic Chemistry and Hydrocarbons (part 1 Intro material) Hein * Best * Pattison * Arena 1

Organic Chemistry • Historically, organic chemicals were defined as those that came from living sources. These chemicals could only originate from a “vital force”. • Friedrich Wohler then synthesized urea from carbon dioxide and ammonia thus disputing the vital force theory • Organic chemistry is now defined as the branch of chemistry dealing with carbon compounds (ie, no vital force needed!) – Certain carbon compounds are not organic: oxides of carbon, metal carbides, and metal carbonates. 2

• Carbon, in combination with hydrogen, oxygen, nitrogen, and sulfur, forms the basis for millions of organic compounds. • Carbon is unique in that it can bond to itself in lengthy chains (a process called catenation). • Organic molecules make up the majority of: 1. 2. 3. 4. 5. 6. 7. 8. Cosmetics and perfumes Nutrients (carbs, fats, proteins) Fuels Fabrics Pharmaceutical products Soaps and detergents Paints, varnishes, dyes Many more! 3

The Carbon Atom: Bonding and Shape 4

• The carbon atom is central to all organic compounds. • A carbon atom usually forms four covalent bonds. – Single bonds • Ex: H 3 C-CH 3 in ethane – Double bonds • Ex: H 2 C=CH 2 in ethene – Triple bonds • Ex: HC CH in ethyne (acetylene) 5

• An organic compound is classified as saturated if it contains only single bonds. – Ex: H 3 C-CH 3 in ethane • An organic compound is classified as unsaturated if the molecules possess one or more multiple carbon-carbon bonds. – Ex: H 2 C=CH 2 in ethene – Ex: HC CH in ethyne (acetylene) 6

The Shapes of Carbon. Containing Compounds • Valence Shell Electron Pair Repulsion theory (VSEPR) – VSEPR theory states that electron pairs repel each other because they have like charges. – The electron pairs will try to spread out as far as possible around an atom. 7

Figure 19. 1 Shows the tetrahedral structure of carbon: (a) a regular tetrahedron; (b) a carbon atom with tetrahedral bonds; (c) a carbon atom within a regular tetrahedron; (d) a methane molecule, CH 4 8

Figure 19. 2 (a) Lewis structure for C 2 H 4. (b) Shape of a molecule with a carbon-carbon double bond. The hydrogens and carbon form the vertices 9 of a triangle. The bond angles are 120°.

In a triple bond, the carbon has only two regions for shared electrons. To be placed as far apart as possible, a linear arrangement is required. 180° H: C: : : C: H 10

Molecular Models • Models are often used in organic chemistry to illustrate molecules. Figure 19. 3 shows molecular models of methane: (a) ball-and-stick model; (b) spacefilling model 11

Figure 19. 4 Models of ethane shows (a) Lewis structure; (b) bond-line drawing; (c) ball-and-stick model; (d) spacefilling model 12

Structural Formulas • In a structural formula, the arrangement of atoms within the molecule is clearly shown. • These structural formulas are often shortened into condensed structural formulas. – e. g. CH 3 CH 2 CH 3 & CH 3 -CH 2 -CH 3 13

Classes of Organic Compounds 14

Classifying Organic Compounds Functional Groups • Organic compounds with similar structures are grouped into classes as shown in Table 19. 1. • The members of each class of compounds contain a characteristic atom or group of atoms called a functional group. These are shown in red in the structural formulas on the table. 15

Functional Groups • A functional group: – is part of a larger molecule. – is composed of an atom or group of atoms that have characteristic chemical behavior. – Chemically, a given functional group behaves in nearly the same way in every molecule it’s part of. 16

Hydrocarbons • Hydrocarbons are compounds that are composed entirely of carbon and hydrogen atoms bonded to each other by covalent bonds. • Fossil fuels – natural gas, petroleum, and coal – are the principal sources of hydrocarbons. 17

Hydrocarbons • Hydrocarbons are classified into two major categories, aliphatic and aromatic. – aliphatic (do not contain benzene ring) • alkanes, alkenes, alkynes, cycloalkanes – aromatic (contain a benzene ring) – See figure 19. 5 18

Saturated Hydrocarbons: Alkanes 19

The alkanes, also known as paraffins or saturated hydrocarbons, are straight- or branched-chain hydrocarbons with only single covalent bonds between the carbon atoms. 20

Homologous Series • When each member of a series differs from the next member by a CH 2 group, the series is called a homologous series. • The members of a homologous series are similar in structure, but have a regular difference in formula. 21

Homologous Series Cn. H 2 n + 2 general formula for open-chain alkanes 22

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Carbon Bonding in Alkanes Figure 19. 6 Schematic hybridization of 2 s 22 px 12 pz 0 orbitals of carbon to form four sp 3 electron orbitals. 24

Figure 19. 7 The tetrahedral nature of sp 3 orbitals: (a) a single sp 3 -hybridized orbital; (b) four sp 3 -hybridized orbitals in a tetrahedral arrangement; (c) sp 3 and s orbitals overlapping to form C-H bonds in methane. 25

Sigma ( ) Bond • A C-C and C-H single bond result from the overlap of sp 3 orbitals. • This type of bond is called a sigma ( ) bond. • A sigma bond exists if the electron cloud formed by the pair of bonding electrons lies on a straight line drawn between the nuclei of the bonded atoms. 26

The valence bonds or points of attachment may be represented in structural formulas by a corresponding number of dashes attached to each atom: 27

• Alkane molecules are non-polar. – Carbon-carbon bonds are non-polar, since they are between like atoms. – Carbon-hydrogen bonds are only slightly polar. – The bonds in an alkane are symmetrically directed toward the corners of a tetrahedron. – In virtue of their low polarity, alkane molecules have relatively low boiling points compared with other organic compounds of similar molar mass. 28

Figure 19. 8 Ball-and-stick models illustrating structural formulas of methane, ethane and propane. 29

Writing the Correct Structural Formula for Propane, C 3 H 8 An alkane contains only single bonds. 30

Isomerism It is possible to write two structural formulas corresponding to the molecular formula C 4 H 10: 31

Isomerism • The phenomenon of two or more compounds having the same molecular formula but different structural arrangements of their atoms is called isomerism. • The various individual compounds are called isomers. – e. g. There are two isomers of C 4 H 10. 32

Figure 19. 8 Ball-and-stick models illustrating structural formulas of butane and isobutane. 33

Isomers are compounds that have the same molecular formula, but different structural formulas. 34

Example for Drawing Isomers 35

There are three isomers of pentane, C 4 H 12. Write structural formulas and condensed structural formulas for these isomers. 1. Write the carbon skeleton of the compound containing the longest continuous carbon chain. C-C-C 2. Complete the structure by attaching hydrogen atoms around each carbon. 36

Isomers of Pentane, C 5 H 12 (continued) • For the next isomer, start by writing a four-carbon chain and attach the fifth carbon atom to either of the middle carbon atoms – do not use the end ones. Add 12 hydrogens to complete the structure. • This is the only four-chain compound that can be written. 37

Isomers of Pentane, C 5 H 12 (continued) For the third isomer, start with a three-carbon chain, attach the other carbon atoms to the central carbon atom, and complete the structure by adding 12 hydrogen atoms: No more isomers of C 5 H 12 can be written. 38